This invention is concerned with the removal of particulate pollutants from a flue gas stream. More particularly, the invention is concerned with the utilization of an Electrified Filter Bed (EFB) and a cyclonic mix chamber for removal of combustible and inert particulate pollutants in a flue gas stream from a wood chip dryer in a plant producing composite board, for example.
As shown in FIG. 1, to produce wood panelboard and other lumber or timber products (e.g., particleboard, waferboard, oriented strand board, oriented strand lumber, chipboard, etc.) a raw feed product, referred to hereinafter as wood chips, must first be dried before it is pressed with binding resin into boards and lumber. The inherent moisture content of the raw feed is typically in the range of 40%-50% by weight and must be reduced to a level of approximately 3%-5% for a satisfactory final product. This reduction in moisture content is achieved by introducing the raw feed into a rotating kiln or dryer 10, which operates at appreciably high temperatures by the continuous flow of hot gas.
The hot gas used to dry the wood chips is conventionally generated in an energy system (later described with reference to FIG. 4) by burning waste wood byproducts, such as bark, sawdust, or the like, in a wood waste combustor (burner). A portion of hot gas derived from the combustor is directed to the gas inlet of the dryer, while the remaining volume of hot gas is introduced to a heat exchanger, which provides thermal oil or steam that serves to power the plant's operation. The output of the heat exchanger is supplied to a collector that separates out some of the dry particulate combustion products. A hot gas stream from the collector is then mixed with the gas supplied from the combustor to the dryer, the mixed gas stream being at a temperature of about 700° F.-1200° F.
The well-mixed hot gas stream passes through the dryer to reduce the moisture content of the wood chips, as previously described. When the raw feed is sufficiently dried, a gas stream from the dryer, containing the dried wood chips, is supplied to a cyclonic product collector 14, which removes the dry wood chip product. A fan 12 supplies the flue gas stream from the collector 14 to a pollution abatement system 16. At the point of exit from the dryer, the flue gas is at an exhaust temperature of roughly 200° F.-300° F., much of the initial thermal energy having been expended in the drying process. The dried wood chips released from the cyclone 14 are homogenized and coated with a binding resin, ultimately to be pressed into panel board, for example, in a downstream process.
The drying operation gives rise to three major types of pollutants, namely, inert fly ash, particulate wood fines, and Volatile Organic Compounds (VOCs). Fly ash is a very fine byproduct of wood fuel combustion, which occurs in the wood waste combustor. Wood fines are small wood fibers that are generated as a result of mechanical agitation in the dryer. Because of their small size, wood fines and fly ash, both of which act as dry dust particles, are not able to be collected by the cyclone product collector 14. The third type of pollutants, VOC's, is derived from wood chips during the drying process, and includes terpenes, isoprenes, resins, and fatty acids. The present invention concentrates on the removal of inert fly ash and combustible wood fines, as described later.
Increasingly stringent environmental regulations require more highly controlled operation of pollution abatement systems. This means that there can be no bypass, or only minimal bypass, of the pollution abatement system. Pollution abatement equipment cannot be taken off-line for maintenance without concurrently terminating critical plant production processes. Prior to current pollution abatement regulations, plants routinely continued to operate wood production processes while pollution abatement systems were off-line for repair and/or inspection. Recent regulations prohibit such practices, and it has become essential that the uptime of the pollution abatement systems be maximized. Optimization of the pollution abatement systems is necessary to provide less downtime, greater production, greater revenue, and a substantial benefit to the environment.
For some time, Electrified Filter Bed (EFB) systems have been used in pollution abatement systems for the removal of particulate in flue gas streams from wood chip dryers. See, for example, U.S. Pat. No. 6,974,494 issued Dec. 13, 2005. Such a system, shown in FIG. 2, is well known and will be described briefly.
In the EFB 20, pollutant particles are given an electrostatic charge, by means of a corona ionizer type device, and are then deposited onto the surface of filter media (e.g., pea sized gravel) in the filter bed. An electrode in the filter bed polarizes the filter media and hence provides caps of positive and negative charge. The electrical force between the charged pollutant particles and the polarized filter media results in effective capture of the pollutant particles on the filter media.
Cleaned, particulate-free, gas exits the EFB and is discharged into the atmosphere by a booster fan 17 and a stack 18. The filter media coated with pollutants is removed from the EFB and is cleaned externally by the use of a pneumatic transport system. Filter media and collected pollutant particles are conveyed pneumatically from the bottom of the EFB system to the top of the system, where the pollutant particles are separated from the filter media via physical impaction using a bounce pad 22. The cleaned filter media are returned to the filter bed for further use, while lighter dust particulate pollutants are carried out via transport air lines and collected in a small collector 24, such as a bag filter or a super efficient cyclone.
Two problems have imposed limitations on the use of EFB units for the removal of particulate pollutants in flue gas streams from wood chip dryers, namely:    1. The inability to use the EFB to clean hot gases from the energy system during downtime of the dryer system for maintenance and repair.    2. The length of time required to preheat the EFB for proper operation.
These problems will now be elucidated by reference, for example, to an Orientated Strand Board (OSB) plant equipped with a direct-heated rotary dryer, which must be taken off-line for repair or maintenance. Because the energy system requires complete combustion of all wood fuel within the system and cannot similarly be taken off-line, the hot gases from the energy system have often been directly expelled into the atmosphere along with significant concentrations of pollutants, which is an unacceptable practice. Adding a special pollution abatement system to control emissions in the event of dryer shut-downs is cost prohibitive and cannot be employed.
It would be highly advantageous to utilize existing EFB systems, installed for abatement of dryer flue gas pollutants, to treat energy system flue gas during the dryer downtime. However, the high temperature (500° F.-600° F.) of the energy system flue gas has prevented such use of the EFB systems. The EFB accumulates considerable amounts of wood fines that spontaneously combust at temperatures in excess of 425° F. in the presence of oxygen. Thus, the use of EFB systems to treat energy system flue gases during dryer downtime would run the risk of fire in the EFB.
The lengthy time required for start-up of the EFB is also a problem, as noted earlier. Typically, when an OSB dryer is equipped with EFB modules, more than 100 tons of gravel is loaded into each EFB module, with some plants incorporating systems with as many as eight EFB modules. Since the EFB units must be pre-heated to a temperature above the water dew point, it has been necessary for the dryer to operate in an idle mode, without a wood chip load, with hot gas running through the dryer and subsequently through the EFB. For large volumes of gravel, this requires many hours (e.g., 2-4 hours) of preheating, during which time no production occurs.
If the plant commences production before proper preheating of the EFB (e.g., after only 15-20 minutes of preheating), large volumes of moist flue gas are introduced into the relatively cold (e.g., less than 150° F.) gravel bed filter, causing water condensation. As the gravel bed heats up, the water evaporates, but localized gravel plugging in various parts of the filter bed will occur. This problem may not manifest itself immediately, but ultimately gravel flow through the filter bed becomes non-uniform, and the condition worsens and eventually causes very high pressure drop across the EFB filter. When this occurs, the EFB must be taken off-line, purged of gravel and cleaned before it can be put back into operation. Such occurrences require many hours of downtime and result in significant production losses.